Once the depth of water becomes large, a production field, and in particular an oil field, is generally worked from a floating support. The floating support generally includes anchor means for keeping it in position in spite of the effects of currents, winds, and swell. It also generally includes means for storing and processing oil and means for off-loading to off-loading tankers. Such floating supports are generally referred to as floating production storage off-loading (FPSO) supports. Numerous variants have been developed such as so-called SPARS which are long floating cigars held in position by catenary anchoring, or indeed “TLPs” which are platforms having tensioned anchor lines, said lines generally being vertical.
Wellheads are generally distributed over the entire field, and production pipes together with lines for injecting water and control cables are placed on the sea bottom converging on a fixed location, with the floating support being positioned vertically thereabove on the surface.
Some wells are situated vertically beneath the floating support and the inside of the well is then directly accessible from the surface. Under such circumstances, the wellhead fitted with its “Christmas tree” can be installed on the surface, on board the floating support. It is then possible from a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required on the well throughout its lifetime. The wellhead is then said to be “dry”.
In order to keep the riser fitted with its dry wellhead substantially in its vertical position, it is necessary to exert upward traction thereon, which can be applied either by a tensioning system using cables and winches or hydraulic actuators installed on the floating support, or else by floats distributed along the riser and installed at various depths, or indeed by using a combination of those two techniques.
The riser as tensioned by these floats is guided, preferably relative to the floating support, by roller guides situated in a plane, preferably a single plane, enabling a riser to be held and guided relative to the floating support. Cable tensioning means acting as guide means can also be used.
FR 2 754 021 discloses a guide device for a riser provided with floats at its head end, the device including wheels enabling the riser to slide vertically, and also enabling it to turn about a horizontal axis so as to guide its horizontal displacements, so that horizontal translation movements of the riser substantially follow those of the floating supports. FR 99/10417 and WO/2001-11184 also disclose an improved guide device having wheels and friction skids disposed radially around the pipe. That device for holding and guiding the portion of a vertical riser that is underwater and near the surface, in particular within a drilling bay, serves to minimize the reaction forces between said riser and the support structure secured to the barge. Finally, various guide systems are known that involve tensioning by cable means.
Since the underwater depth of certain oil fields exceeds 1500 meters (m), and can be as great as 3000 m, the weight of risers over such depths requires forces to be used for holding them in position that can reach or exceed several hundreds of (metric) tonnes. “Can_type” buoyancy elements are used that are added to underwater structures, mainly to risers connecting ultra-great depths (1000 m-3000 m) to the surface. The underwater pipe then consists in a rising column having an underwater pipe assembled to at least one float comprising a coaxial can surrounding said pipe with said pipe passing therethrough.
The floats in question are of large dimensions, and in particular they have a diameter of more than 5 m, a length of 10 m to 20 m, and possess buoyancy that can be as great as 1000 tonnes, and in general they are disposed as a string one beneath another.
The float and the pipe are subjected to the effects of swell and current, but since they are connected to the FPSO at the surface, they are also indirectly subjected to the effects of wind. This gives rise to lateral and vertical movements that are large (several meters) for the riser-float-barge combination, particularly in the zone that is subjected to swell. These movements generate large differential forces between the riser and the float. In addition, the bending applied to the riser leads to bending moments that are extremely large in the zone where there is a change in second moment of area, as arises whenever there is a connection between the riser and a float.
In order to minimize the forces generated by current and swell and acting on the riser-float combination, floats are generally circular and are installed coaxially around the riser.
In addition, floats are generally secured to the riser in such a manner that the riser-float connection provides sealing to said float so that it can confine a filler gas. The solution that is commonly used consists in embedding the float in the riser by welding, both at the top and at the bottom of the riser. Large amounts of reinforcement are added to ensure that the connection is sufficiently strong.
At such a connection between the riser and a float, the second moment of area changes considerably on passing from the section of the riser to the section of the float.
These large variations in second moment of area lead to stresses being unevenly distributed, thereby generating very localized zones in which stresses can become unacceptable and can lead either to phenomena of sudden rupture, or else to phenomena of fatigue that in turn lead to cracks appearing, followed by collapse. These localized stresses require transition pieces to be used to reinforce the weak zone, said pieces generally being conical in shape and of large dimensions, and being referred to as “tapered joints”. In some circumstances, these pieces can be as much as 10 m long, and even under the best of circumstances they require the use of very high performance steels. However, it is often necessary to make use of titanium which is 5 to 10 times as expensive as the best steels. In addition, such pieces are generally complex in shape and they need to be made with extremely high quality so as to provide the expected service throughout the lifetime of the equipment, which lifetime can commonly exceed 25 years.
U.S. Pat. Nos. 3,952,526 and 3,981,357 disclose junction systems between float-tanks and risers, in which use is made of parts made of elastomer material.
Those buoyancy systems make it possible to reduce the tensioning system situated on board the floating support, and they are generally distributed over a major fraction of the water depth, and in addition they present small buoyancy, generally a few hundreds of kilograms (kg) or possibly one or two tonnes.
The junctions are situated in the top portions of the floats, with the bottom portions of the floats generally being open. Such devices can transfer loads corresponding to lightening only a short length of the pipe, but they are not suitable for floats that are intended to support a very great length of riser, e.g. 500 m to 1000 m or even more, either alone or with the help of additional tensioning systems secured to the floating support, where such lengths are to be found in very deep offshore oil fields, i.e. at depths of more than 1000 m. The buoyancy needed to achieve tensioning solely by means of floats requires considerable forces to be transferred vertically and transversely, and said vertical forces, when applied to the head of the riser, can reach several hundreds of tonnes, and can in particular lie in the range 300 tonnes to 500 tonnes.
In WO/2001-04454 in the name of the Applicant, there is disclosed a novel type of junction between the riser and the can that serves to support and transfer high loads, while mitigating the drawbacks of the above-mentioned floats assembled around said pipe by the pipe being embedded therein.
Those (riser-float) junction means are simple, flexible, and mechanically reliable, and they reduce the phenomena of fatigue and wear due to the stresses acting on the junction which is subjected to loads of several hundreds of tonnes.
More particularly, patent WO/2001-04454 in the name of the Applicant describes a string of floats surrounding a vertical riser, each of said floats being fitted at at least one of its ends with a flexible joint comprising laminated abutments serving not only to provide sealing and to transfer loads, but also to decouple the second moment of area between the structures of said float and of said riser, so that there is practically no longer any zone in which stresses are concentrated at the transition between said float and said riser, thereby making it possible to reduce the complexity of the structure of the connection and also its own weight, thus significantly increasing the efficiency of the float, i.e. its buoyancy compared with its own weight.
Still more precisely, WO/2001-04454 describes junction devices between the riser and the float that comprise laminated abutments made up of layers of elastomer sandwiched between rigid reinforcements, being supported by plates comprising a first plate secured to the pipe and a second plate secured to the float, with said rigid reinforcements and elastomer layers being:                either in the form of superposed washers;        or in the form of tubes or cylinders that are coaxial and adjacent.        
In WO/2001-04454, the bottom-to-surface connection is thus continuous in the zone where the float is installed, and the flexible junction serves to decouple the second moment of area of said float from that of said riser.
Current acts over the entire height of the riser, from the sea bottom up to its surface, but swell acts only in a zone close to the surface and decreases in substantially exponential manner so as to become practically zero at a depth of about 80 m to about 120 m. Thus, when using a string of mutually independent floats as described in WO/2001-04454, the top floats are subjected to considerable forces both laterally and vertically since the effects of swell are very large in zones close to the surface, while the bottom floats are subjected to much less stress. The unit dimensions of the floats are limited since they must be capable of being handled on board the barge and then introduced into the derrick in order to be lowered through the drilling bay. Thus, in very great depth, e.g. of 2000 m to 3000 m, the weight of the riser is such that a large number of floats is required, e.g. four or five floats presenting total buoyancy of 400 tonnes to 500 tonnes and extending over a height of about 100 m.
Each of the floats needs to be fitted with laminated abutments so as to minimize the transfer of stresses to the vertical riser which constitutes a highly critical element of the bottom-to-surface connection since it must be capable not only of withstanding very high tensions, but also it must be capable of withstanding the bursting pressure created by the fluid it transports, and also the implosion pressure created by the sea water.
This buoyancy, which is distributed as a multitude of independent floats, requires numerous laminated abutments to be used, each being of high cost. In addition, swell creates differential forces between pairs of adjacent floats, which forces are in addition to the considerable forces to which the riser is subjected at each discontinuity between the riser and a float.
It is thus desired to minimize the number of floats, but when the floats take on large dimensions, the transition zone between the bottom end of the float and the riser concentrates considerable horizontal thrust forces, thereby requiring said riser to be reinforced by a transition piece constituted by a conical forging of great length that is very difficult to fabricate and therefore very expensive, since it is generally made of very high performance metal, such as titanium. When there is only one float, it needs to be enormous when the depth of water is large, and the risk of failure associated with the transition piece then becomes very high and therefore unacceptable because of the high risk of pollution in the event of said bottom-to-surface connection failing or rupturing.
Furthermore, the entire riser behaves like a tensioned cord between the sea bottom and the point situated on the axis of the guide system relative to the floating support, and this leads to vibratory phenomena of the guitar-pendulum type. Water moving in the depth of the water creates drag effects on the structure of the riser and its floats, thereby generating large forces in varying directions, together with vibratory phenomena created by turbulence in the moving water separating from the riser.
Patent WO/2001-53651 in the name of the Applicant describes a device for stabilizing a bottom-to-surface connection of the type comprising a riser tensioned by a float, said tensioned riser being guided at a surface support, preferably in a single plane. Said stabilization device serves to avoid vibratory phenomena of the guitar-pendulum type appearing, thus avoiding localized accumulations of fatigue appearing in the steel as are usually to be encountered in the transition zone between the bottom of the float and the portion of the riser situated immediately below said float, said fatigue phenomena leading rapidly to cracking and then to rupture of the installation.
Nevertheless, that stabilizer device does not make it possible to avoid having recourse to reinforced transition pieces, generally conical forgings of steel or titanium, where titanium presents particularly high performance in terms of resistance to fatigue, but is particularly expensive because of its raw material cost and its difficulty of manufacture.